The brightest comet of the month will probably not be observed directly by anyone from Earth as 96P/Machholz will remain too close to the Sun after its October perihelion. The brightest nighttime comet should be C/2017 O1 (ASASSN) as it fades from 9th to 10th magnitude. The second and third brightest nighttime comets should be returning periodic comets 24P/Schaumasse and 62P/Tsuchinshan. While 24P is predicted to be brighter (at magnitude 10.5 vs 11.5), observations during October suggest that 62P may be the one that brightens to magnitude 10.5 with 24P being fainter.

Evening Comets

29P/Schwassmann-Wachmann 1 [Perihelion on 2019 March 7 at 5.77 au]

Outburst prone 29P/Schwassmann-Wachmann has been quite active this year. The comet has been consistently observed between magnitude 11 and 14 since September. How bright it will be this month depends on whether it experiences another outburst. This is one comet where rapid changes in brightness and coma morphology make day-to-day (and sometimes even hour-to-hour) monitoring worthwhile. CCD imagers are especially asked to keep a watch on this enigmatic object as it slowly drifts near the Capricornus/Aquarius border in the evening sky.

C/2017 O1 (ASASSN) [Perihelion on 2017 Oct 13 at 1.50 au]

As November starts, the brightest comet in the night sky is C/2017 O1 (ASASSN). Due to this comet’s diffuse and large coma, a large range of magnitude estimates has been reported. Binocular observers have reported the comet to be between magnitude 8.5 and 9.0 while larger telescope observers have reported the comet to be as faint as magnitude 10.5. In my 30×125 binoculars, ASASSN is large (7-8′), very diffuse and hard to detect as anything more than a faint brightening against the background sky. It could be easy to miss under a Moon-lit or light polluted sky. This month the comet should fade as it is moving away from the Sun (1.52 to 1.64 au) and Earth (0.75 to 0.95 au). For those willing to take a look at ASASSN, it will be located in the northern circumpolar sky as it moves through Camelopardalis (1-15) and Cepheus (15-30).

Morning Comets

24P/Schaumasse [Perihelion on 2017 Nov 16 at 1.21 au]

Short-period comet 24P/Schaumasse is making its 11th observed return since its discovery back in 1911 by French astronomer Alexandre Schaumasse. Perihelion occurs this month on November 16 at 1.21 au and closest approach to Earth on the 21st at 1.46 au. The comet was expected to peak at 10-11th magnitude this month. Recent visual observations show it to be running 1-2 magnitudes fainter than predicted. At the end of October, the comet was between 12-13th magnitude. Whether it brightens to 10th magnitude remains to be seen but it is very possible that Schaumasse may not get brighter than 11-12th magnitude. this month as it moves through Leo (Nov 1-8) and Virgo (8-30). Its next return in late 2025/early 2026 will be better with a close approach to Earth of 0.59 au.

62P/Tsuchinshan [Perihelion on 2017 Nov 16 at 1.38 au]

While 24P/Schmaumasse underperforms, 62P/Tsuchinshan appears to be a brighter than expected. Discovered photographically in 1965, 62P/Tsuchinshan (Chinese for Purple Mountain which is the name of the observatory where the discovery was made) is making its 9th observed return. The comet was observed between magnitude 10.5 and 11.0 at the end of October. It should brighten by another 0.5 to 1.0 magnitude as it reaches perihelion on Nov 16 at 1.38 au. This brightness would match 62P’s performance at its return in 1985. Looking ahead, its next return in late 2023/early 2024 will be even better with a lower perihelion distance (1.26 in 2023 vs 1.38 au this year) and geocentric distance (0.50 au in 2023). This month, its heliocentric distance stays around 1.38 au and its geocentric distance drops from 1.41 to 1.29 au as it moves through Leo (Nov 1-26) and Virgo (26-30) in the morning sky.

New Discovery

The last Comet Section post introduced two recent discoveries that should become bright enough for visual observers, C/2017 T1 (Heinze) and C/2017 T2 (PANSTARRS). The ATLAS survey, which also discovered C/2017 T1, has found another comet which should become bright in 2018. C/2017 T3 (ATLAS) was discovered on October 14 at 18th magnitude. The comet is currently in the far northern sky in Cepheus at a distance of 3.8 au from the Sun. It comes to perihelion on 2018 July 19 at 0.83 au. Unfortunately, it does not get too close to Earth (1.35 au on August 1) though it is possible that this dynamically old long-period comet will brighten to magnitude 9 or 10 around perihelion.

Mike Olason imaged two of the recent discoveries (C/2017 T1 and C/2017 T2) with his 11″ SCT and SBIG STF-8300M CCD from Colorado.